A known structure of a lightning arrester with an ESE device can include an active portion which is mounted on a tip if the lightening arrester and is connected to the ESE device for applying voltage pulses to the active portion; a down conductor to be grounded; and a dielectric portion through which the active portion is mounted on the down conductor, the active portion being separated from the down conductor by an air space between them to form a spark gap with air through which the lightning current passes from the active portion to the down conductor.
Once installed on the site to be protected, ESE lightning arresters are placed in an external environment. As a result, the dielectric properties of the spark gap with air formed between the active portion and the down conductor may be degraded by environmental conditions.
For example, rainwater can be deposited on the dielectric portion of the ESE lightning arrester or even be suspended as droplets in the air of the spark gap of the lightning arrester. As a result, the level of electrical isolation between the active portion and the down conductor of the ESE lightning arrester is decreased due to the conductivity of rain water, which has the effect of degrading the performance of the ESE lightning arrester. In the limiting case, the active portion and the down conductor are short-circuited, preventing the application of voltage pulses to the active portion of the lightning arrester, in which case the ESE lightning arrester only operates as a simple Franklin rod.
In addition, air pollution can affect operation, such as when the ESE lightning arrester is mounted near factory chimneys, the chimneys of factories being frequently used as a high point of buildings for the layout of lightning arresters. The particles of pollution can remain suspended in the atmospheric air and therefore in the air space of the spark gap with air of the ESE lightning arrester, but they can be deposited on the dielectric portion of the ESE device. Such particles of pollution may also have some electrical conductivity leading to the deterioration of the insulation level of the spark gap with air of the ESE lightning arrester. It is also possible for insects to come to nest in the space between the active portion and the down conductor of the ESE lightning arrester forming the spark gap with air.
French patent application FR-A-2799585 discloses an active portion of an ESE lightning arrester which includes a metal bell-shape open at the bottom. Inside the bell-shape is a spark gap which is formed between it and a base connected to the down conductor. Similarly, the base is insulated from the bell-shape by an insulator which is also placed inside the bell. Thus, the spark gap and the insulation are protected to some extent from the environment.
However, this protection may not be satisfactory because water droplets can reach the spark gap and the insulation despite the bell, such as in the case of thunderstorms with swirling winds. The bell-shape does not prevent the penetration and deposition of particles such as in cases of air pollution, or the penetration of insects.